Detection of binaries in the core of the globular cluster M15 using calcium emission lines

M15 is the prototypical collapsed-core globular cluster. Having undergone collapse, its core is believed now to be expanding, with energy for the re-expansion provided by binary stars, which turn gravitational potential energy into kinetic energy¹. Because these binary stars are generally more massive than single stars, they will have settled to the centre of the cluster². We report here that several of the stars at the core of M15 show Ca II H- and K-line emission, characteristic of young, rapidly rotating stars and close binaries³. We argue that the emission from M15 comes from primordial binaries, in which a period of spin-up has led to magnetic field generation by enhanced dynamo action, which in turn causes heating of the stellar chromospheres. If this interpretation is correct, the Ca H and K emission may provide an important diagnostic tool of the binary population in cluster cores, and thus of the cluster dynamics.